I'm asking because I cannot find it anywhere in the docs.

As a side note, this project is in serious need for proper documentation. Couldn't hurt to go the wiki way here and let users add to it if you guys don't have the resources

We are working hard on the documentation now! Unfortunately we neglected it for a few years as we focused on feature development, so it has some catching up to do.

Prince supports (much of) Ecmascript 3, if I am correctly interpreting the somewhat tangled history of Ecmascript versions.

The documentation on JS/ECMA has been updated ("most of ECMAScript 5th edition" is the current answer).